The invention relates to a ship of the kind used in the production or test production respectively in connection with exploitation of small fields of oil/gas below seabed level, whereby the production may take place by means of a so-called STL-buoy or corresponding device formed with a preferably upward tapering, conical, first turret that the ship is arranged to receive by a lower, complementarily formed recess.
Thus, it is known to produce oil from ships via such a buoy, which is a buoyancy body held in a suitably submerged condition by means of mooring hawsers anchored to the seabed. Internally the buoy is formed with at least one through channel or bore connected to a riser constituted by tubing leading to one or more subsea wells.
A ship of the kind mentioned is a production ship which is only adapted to co-operate with such a buoy, and for this purpose the ship is provided with a swivel device interconnecting the pipeline outlet(s) of the buoy and at least one stationary pipeline aboard the ship, leading to a cargo compartment for oil. Such production ships are not suitable for application in test production of wells, because of, among other things, said swivel device is only working in connection with the buoy and not in connection with so-called test tubings. Such test tubings are not arranged to be connected to the pipeline outlet(s) of the buoy through the swivel device, but have a free vertical extension from a rig aboard the ship to the buoy to be tested. When a production ship is pivotally connected to a buoy via said swivel device, the vertical axis of rotation of the buoy will form the axis of rotation of the ship. In this case the buoy constitutes the "turret" of the ship. When such a ship is to be used in test production, said ship will lack an essentially stationary turret about which it may turn dependent on wind, sea and water currents.
According to the present invention, there has been aimed at providing a ship, which, through relatively simple structural modifications, will be well suited for production in connection with buoy as well as for test production. In this way a more flexible use of the ship is achieved, the ship thus to be used as production ship in connection with a buoy in one particular place in one period, and then, in another period and in another place, to be used as test production ship.
According to the invention this has been achieved by means of a combination vessel in the form of a production/test production ship which distinguishes itself by the features defined in the characterizing part of the claims.
According to the invention, immediately above its downward open recess for the reception of a buoy, this combination vessel is configured with a preferably through opening extending vertically upwards, in which is arranged a (second) turret which is free to rotate in the ship about a vertical axis, about which the ship may pivot depending on wind, sea and currents when it is used in connection with test production. This second turret, the buoy is called the "first" turret, serves for passing test tubing, which is connected aboard the ship, through a swivel device, to at least one stationary pipeline which may lead to a cargo compartment for oil. The test tubings mainly do not move during the test production operations, while the ship may turn freely about the vertical axis of the second turret. When a production/test production ship provided with a turret (second turret) of its own is to be used as a production ship in connection with a buoy, the latter--the first turret--is securely connected to the second turret, at the same time as the channel(s)/bore(s) of the first turret is(are) connected to the pipeline(s) of the second turret. In these connecting operations it is an advantage if the second turret is arranged liftable/lowerable and may be brought to adopt the lifted position during at least the preliminary work, as the actual connecting takes place when the first turret (the buoy) is brought into place in the downward open recess of the ship, and the second turret has been lowered into its second position.
Thus, the ship has two functions, namely (a) as a test production ship when exclusively the second turret forms the axis of rotation of the ship and the through passage for pipeline(s), and (b) as a production ship when the second turret is securely connected to an underlying buoy forming the first turret, and in which the connected first and second turrets together form the axis of rotation of the ship and the passage for said pipeline(s) extending down into the sea. In both cases the further transport of oil takes place by means of a stationary pipeline(s) via said swivel device, which will normally be positioned on top of the second turret.
When the second turret is used in test or trial production, the ship is held in position by so-called dynamic positioning, i.e. without the use of anchor hawsers for anchoring to the seabed.
Objects, advantages and features of the present invention will appear from the following description in conjunction with a non-limiting example of a preferred embodiment of a production/test production ship configured in accordance with the invention.